Hillary Clinton and Barack Obama face nine more contests in their marathon nominating battle, but both know that there is just one that has the power and significance to decide the race: their next showdown in Indiana.

The May 6 contest in another large, Midwestern state with similar demographics to Ohio and Pennsylvania has emerged as the last and possibly the decisive battleground of the Democratic race. It is the only remaining primary where the result is uncertain and polls have the two candidates essentially tied.

For Mrs Clinton Indiana looms as yet another must-win state if she is to keep her candidacy alive, just as her victories in New Hampshire, Ohio and now Pennsylvania were make-or-break moments for a campaign that is used to near-death experiences.

The Hoosier State presents Mr Obama with perhaps his last chance to see if he can use his huge financial advantage over Mrs Clinton to find a way of persuading white, working-class voters to back him, a critical general election voting bloc that has so far stubbornly refused to be swayed by his message of hope and generational change.

His failure to win over these “Reagan Democrats”, who again turned their backs on him in Pennsylvania on Tuesday, is emerging as the greatest threat to his candidacy.

Mr McCain turned the knife yesterday, telling a crowd in Kentucky that Mr Obama’s comments that working-class Americans cling to guns and religion were elitist.

Both candidates held events in Indiana yesterday. Mr Obama hurried there on Tuesday night before his defeat in Pennsylvania had even been announced.

The Hoosier State, he says, “may end up being the tie-breaker”. At his primary night rally in Evansville, he declared: “Now it’s up to you, Indiana.”

Asked if a defeat in Indiana would be a terminal blow to Mrs Clinton’s campaign, Howard Wolfson, her communications director, conceded that the contest was critical. She will campaign there for three of the next four days.

The other state to vote on May 6, North Carolina, heavily favours Mr Obama because of its large proportion of African-Americans. A double defeat in two weeks time would reshape the race yet again, and could trigger a rush of uncommitted super-delegates Mr Obama’s way, many of whom are anxious to see an end to the increasingly destructive contest.

Indiana is the last battleground for the two candidates because the results in nearly all the other remaining contests are almost foregone conclusions. Mr Obama is expected to prevail in North Carolina, Oregon, Montana and South Dakota; Mrs Clinton in West Virginia, Kentucky and Puerto Rico. Guam, a territory that votes on May 3, only offers four delegates.

At first glance the demographics of Indiana, with its predominately white population, industrial base and rural, culturally conservative heartland, should favour Mrs Clinton – just as the same factors contributed to her victories in Pennsylvania and Ohio. Up to 60 per cent of the electorate on May 6 will be women, another advantage for her. She has also been endorsed by Evan Bayh, the state’s popular US senator and former Governor, who has been campaigning energetically on her behalf.

He is a figure being talked about as a possible running-mate for the former First Lady if she wins the nomination. Much of the state’s Democratic Establishment has also backed her.

Yet Mr Obama enjoys advantages in Indiana that he did not have in Ohio and Pennsylvania. It borders his home state of Illinois. More than 20 per cent of Indiana, especially the populous northern region where is the strongest, is in the Chicago media market. He has won every state bordering his own so far: Iowa, Wisconsin and Missouri.

Unlike Pennsylvania, Indiana is an open primary, meaning that Republicans and independents can vote in the Democratic contest. Mr Obama has done well in such elections, and struggled when only registered Democrats can participate.

Mr Obama will also be looking for another huge turnout of blacks in Indianapolis, the state’s largest city, where about 30 per cent of the electorate is African-American. Indiana also has a significant number of large universities and students have backed him heavily. More than 40,000 new voters have registered for the primary, the majority of whom are young, first-time voters. Mr Bayh said that Mrs Clinton “starts a little behind” in Indiana, although the latest polls gives her the smallest of advantages.

For Indiana itself, the primary is the first time in 40 years that it has played such a significant role in a primary contest, since Bobby Kennedy contested the state against Eugene McCarthy in 1968.

Times Recommends

• Please help: maimed children of war want their lives back
• Cabal seizes diamond riches to buy power
• White House gatecrashers got to the president